Known back-pressure valves may have a diametrical bridge in the circular opening of the mask. An axial stud affixed in the center guides the sealing disc having in its center an appropriate bore. A conical spring abuts with its large end against the sealing disc and with the small end against a head of the stud. Although there is provided a clearance between the stud and the bore, it is seen that the sealing disc can move essentially only in an axial direction. The flow resistance and hence the loss of pressure with an open back-pressure valve is relatively important in the pipe system concerned.